1. Field of the Invention
The present invention relates to a data reproduction apparatus, and more particularly, an apparatus for detecting and correcting phase difference between signals and an apparatus for detecting a header of data.
2. Description of the Related Art
There is a desire for transferring and recording vast amounts of information since considerable amounts of information are handled in many fields, for example, in data communication apparatuses (e.g. mobile phones, satellite communication) and data recording/reproduction apparatuses for optical disks and magnetic disks.
With a conventional data reproduction apparatus for optical disks or magnetic disks, for example, a synchronizing signal indicating the header of recorded data is first required to be reproduced for restoring reproduced data into binary information data. The synchronous signal is reproduced by using a PLL (Phase Locked Loop) circuit. The simple principle of the PLL uses a VCO (Voltage Control Oscillator) for generating a clock signal, which serves as a reference for digitizing the reproduction signal, comparing the phase of the clock signal and the phase of the reproduction signal, and adjusting the phase of the clock signal generated from the VCO so that the phases come to have a predetermined relation.
Therefore, during phase lock of the reproduction signal, phase selection, and frequency synchronization, an initial phase detector is provided to detect phase difference between the reproduction signal and a sampling clock for sampling it.
FIG. 1 is a block diagram showing a reproduction system of a conventional data reproduction apparatus. The reproduction system of the data reproduction apparatus shown in FIG. 1 mainly includes: an optical head 102 which irradiates an optical beam 103 to an optical disk 101, receives light reflected from the optical disk 101, and converts this into electric signals; an AGC 104 and an analog equalizer 105 which receive signals output from the optical head 102; an A/D converter 106; a digital waveform equalizer 107; a decoder 108, a phase difference detector 110; a delay 111; a fine clock mark (FCM) detector 112; a PLL 113, and an address mark detector 114.
The reproduction system of the conventional data reproduction apparatus shown in FIG. 1 reproduces the MO signal 120 reproduced from the optical disk 101 into user data 130 by employing an external clock method.
Here, the external clock method, in the reproduction system of the conventional data reproduction shown in FIG. 1, does not generate a clock for reproducing data from the MO reproduction signal 120 itself including information data, but it is rather a method which generates a reproduction clock 124 using the PLL by synchronizing a signal (in FIG. 1, tangential push-pull signal 121) reproduced from a particular clock mark embedded in a medium to a signal 123 detected by the fine clock mark detector 112 in the PLL 113.
FIG. 2A is a drawing showing a fine clock mark (FCM) 201 recorded on the optical disk 101. Furthermore, FIG. 2B is a drawing showing a disk format of only a single extracted track. Furthermore, FIG. 2C is a drawing showing a principle generating the external clock signal 124 from the fine clock mark signal by using the PLL 113.
The fine clock mark (FCM) 201 shown in FIG. 2A is reproduced by applying the optical beam 103 from the optical head 102. FIG. 2B is a drawing showing the fine clock mark (FCM) reproduced by the optical head 102 from the optical disk 101. As shown in FIG. 2C, the reproduced fine clock mark (FCM) 201 is detected by the fine clock mark detector 112 and is output from the fine clock mark detector as an FCM detection signal 123 having the fine clock mark (FCM) extracted in a pulse-like manner. The FCM detection signal is multiplied by the PLL 113, to thereby generate the clock signal 124.
Meanwhile, the MO signal reproduced from the optical disk 101 by the optical head 102 has its amplitude controlled by the AGC 104, then has its waveform equalized by the analog waveform equalizer 105, and then is sampled by the A/D converter 106 by using the above-described reproduced clock.
In the external clock method, however, sampling clock frequency information for sampling employs the fine clock mark reproduced from the medium for reproducing data. Accordingly, since the detection system for detecting the fine clock mark 123 and the detection system for detecting the MO signal 120 are physically different, there is a phase difference between the reproduced clock 124 and a signal 125 resulting from the waveform-equalization of the MO signal 120. Such phase difference is required to be adjusted for achieving optimum sampling of the reproduction signal.
FIG. 3 is a drawing showing a reproduction signal processing part of the reproduction system of the conventional data reproduction apparatus shown in FIG. 1, in which the above-described phase difference is adjusted by using the reproduction signal processing part. In FIG. 3, like components are denoted by like numerals as of those of FIG. 1.
As shown in FIG. 3, a sampling signal, being waveform-equalized by the digital waveform equalizer 107, is sent to an ML (Maximum Likelihood) decoder 108. Then, the above-described phase difference is detected by the phase difference detector 110 based on the signal decoded by the ML decoder 108. Then, the delay of the clock output from the PLL 113 is changed by the analog delay in accordance with phase difference information obtained from the phase difference detector 110. As a result, the phase of the external clock 126 is synchronized to an optimum phase for sampling the MO signal 125 being waveform-equalized by the analog waveform equalizer 105, and waveform-equalized MO signal 125 is sampled with the external clock 126 by an appropriate phase. Accordingly, the waveform-equalized MO signal 125 is sampled with an optimum sample clock.
Meanwhile, owing to the remarkable advances in data signal processing methods using, for example, encoding technology, data can be demodulated and decoded even in a case where the Signal to Noise (SN) ratio of the reproduction signal is low. However, it is still difficult to execute phase lock adjustment between an optimum sampling point of a reproduction signal and a clock for sampling the reproduction signal, for example, in a case where the SN ratio of the reproduction signal is low or a case where the signal amplitude changes. In order to execute phase adjustment with more accuracy, the conventional example has a problem of requiring a large amount of space on the information medium for phase adjustment. In addition, there is also a problem of increasing size of the detection circuit for detecting phase difference with high accuracy.
Furthermore, conventionally, the header of data is detected with an ODC (Optical Disk Controller) after decoding is executed with a PRML (Partial Response Maximum Likelihood) detection method. However, in employing turbo code, which is being considered for application to magnetic recording systems, the header of reproduced data is required to be obtained with accuracy prior to decoding of data for executing block encoding and decoding. Therefore, in a system using the turbo code, the header of data reproduced from an information medium is required to be reliably detected without deviation of data of a single clock.